01․ Making and breaking currents of 3 phase AC circuit breakers in power system are respectively in what form?
The instantaneous maximum or peak value of the current flowing through the circuit breaker at the instant of closing of circuit breaker is called making current.
Making current = 2.55 * Breaking current
The RMS value of short circuit fault current passing through the circuit breaker at the time of opening of circuit breaker is called breaking current
02․ In a 220 kV system, the inductance and capacitance up to the circuit breaker location are 25 mH and 0.025 µF respectively. The value of resistor required to be connected across the breaker contacts which will give no transient oscillations, is
The value of resistor required to be connected across the breaker contacts which will give no transient oscillations R is
R = 0.5* √(L/C)
R = 0.5 * √(25*10-3/(0.025*10-6))
R = 500 Ω
03․ Which one of the following relays has the capability of anticipating the possible major fault in a transformer?
Buchholz relay is the gas detector relay used to protect transformers which operates when the oil level in the conservator pipe of a transformer is lowered by the accumulation of gas caused by a poor connection or by incipient breakdown of insulation.
04․ Mho relay is usually employed for protection of
Mho relays are usually employed for protection of long transmission lines only, as mho relay operation is not effected by power swings.
Long transmission lines ----------- Mho relays
Medium transmission lines ------- Impedance relays
Short transmission lines ----------- Reactance relays
05․ Normally Z bus matrix is a
Z bus algorithm or matrix is used for the fault analysis. Inverse of Y bus matrix gives the Z bus matrix, But Z matrix is a full matrix even though Y bus is a sparse matrix. More time is required for inverse of Z bus matrix if the size of the matrix is more than three.
06․ A generating station has a maximum demand of 100 MW, a load factor of 60% and a plant capacity factor of 50%. The reserve capacity of the plant is
Reserve capacity Rc = Plant capacity(Pc) - Maximum power(Pmax)
Rc = Pmax*(load factor - capacity factor)/capacity factor
Rc = 100*0.1/0.5 = 20 MW
07․ The rating of fuse wire is always expressed in
The current flowing through the fuse wire is same as the load current which is consumed by the electrical load. That load current only decides the rating of the fuse. Hence, the rating of the fuse is expressed in Amps.
08․ Lightning arresters are used for
A lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. For protecting the sub station equipment from the external lightning, lightning arresters are provided on both ends of the sub station. This is the first protecting device in the sub station.
09․ Lightning arresters are made up of which material?
Lightning arresters are made up of metal oxides like silicon oxide, zinc carbide which has non linear characteristics and it offers high resistance for normal supply voltage and low resistance for the lightning voltage. So that lightning is diverted to the earth.
10․ Which of the following transformer is used in sub station for operating a protective relay?
A capacitor voltage transformer (CVT ), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay.
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